1. Field of the Invention
This invention relates to an alternator for a vehicle such as a passenger automotive vehicle or a truck.
2. Description of the Related Art
There have been various proposals to provide small-size high-power alternators for vehicles.
Japanese published unexamined patent application 6-46550 discloses a way of improving the power generation performance which uses a permanent magnet.
To meet demands for downsizing, it is necessary to use a small-size cooling fan in an alternator. The rate of air flow generated by the small-size cooling fan is relatively low. On the other hand, a high-power alternator generates an increased rate of heat. Thus, a high-power alternator with a small-size cooling fan tends to have the problem of a temperature rise.
Accordingly, to provide a small-size high-power alternator, it is necessary to remove the problems caused by a temperature rise. Especially, it is necessary to implement effective heat radiation from electric conductors in a power generating stator under small size restrictions.
Japanese published unexamined patent application 7-194060 discloses that water having a high heat radiation efficiency is used as coolant for an alternator. The alternator in Japanese application 7-194060 needs pipings for the cooling water, and a water jacket in the alternator body. Accordingly, the alternator in Japanese application 7-194060 tends to be heavy and large in size. Thus, the alternator in Japanese application 7-194060 does not meet demands for downsizing.
General prior-art air-cooling techniques include a step of decreasing the temperature of coil end portions of electric conductors in a stator. The coil end portions are also referred to as the bridge portions.
Japanese published examined patent application 4-24939, Japanese published unexamined patent application 63-59744, Japanese published examined utility model application 1-27406, and Japanese published unexamined patent application 57-132743 disclose improvements of bridge portions.
According to these prior-art air-cooling techniques, pieces of an electric conductor which form bridge portions are arranged so that cooling air can efficiently flow therebetween and improved heat radiation can be attained. In the prior-art air-cooling techniques, bridge portions of electrical conductive wires are partially spaced from each other, and are in a flat and regular arrangement as a whole. In the prior-art air-cooling techniques, the bridge portions interfere with paths for the flow of cooling air, and hence offer great resistances to the flow of cooling air. An impregnation agent for fixing the bridge portions covers the surfaces thereof, and provides an increased resistance to the cooling air. Accordingly, the prior-art air-cooling techniques have a poor cooling performance.
In a general prior-art structure, an electric conductor in a stator is coated with an insulating film, and an impregnation agent for fixing the electric conductor is applied onto the insulating film. It is well-known that the insulating film considerably deteriorates the heat radiation from the electric conductor. Since sufficient insulation is necessary, it is impractical to remove or thin the insulating film.
In a prior-art structure, the heat resisting property (the allowable temperature) of electric conductors coated with insulating layers is determined by the heat-deterioration temperature of the insulating layers in a region where the electric conductors overlap each other. Thus, it is difficult to improve the heat resisting property (the allowable temperature).